This invention relates to improvements in ground fault detection and protective systems.
The need for a complete circuit as a prerequisite to electrical current flow is satisfied by the provision of two conductive paths between a power source and its load. Many electrical power distribution systems, and almost all utility distribution systems, employ a pair of electrical conductors one of which is grounded to the earth. That is called the neutral line. The other conductor, the ungrounded one, is commonly called the "hot" line. A variety of fuses and circuit breakers and magnetic blowouts and thermally actuated switches are available for incorporation in such power lines to protect the line and the source from the current overload that results from short circuits and overloads. It is also common to include a third conductor in the system to serve as a ground wire. It is connected in parallel with the neutral line and is connected to the enclosures and frames and other metallic components of electrical appliances, motors, and other electric power utilization apparatus. In other systems, the neutral wire is connected to the metal case or frame and performs the dual function of serving as a ground wire and power conductor. Whatever the arrangement, the reason for connecting the frame or case of an electrical appliance to the power conductor is to ensure that the protective fuse will be blown or the protective circuit breaker will be opened, in the event that the hot line should become grounded to the frame or the case. If ungrounded exposed metal members of an appliance should come into contact with the hot line, the case or frame would become "hot". It would present a shock hazard to one who might touch the exposed metal at the same time that he was connected to ground or earth.
While the practice of grounding the enclosure and major metal parts of electrical appliances has enhanced their safety, it has introduced another safety hazard. If the appliance user, or some other person or animal, should touch a hot line while simultaneously touching the grounded case or frame, he may experience an electrical shock. The fuse, or circuit breaker, that protects the line and power source will not protect a person who touches both the case and the hot line. They are operative to disrupt power only after the flow of an excessive current.
An unwanted circuit between the hot line and the grounded frame of an appliance, or between the hot line and ground, is called a "ground fault." Apparatus which can detect ground faults and supply a signal for use in interrupting electrical power to the fault is called a "ground fault sensor." Apparatus which can both detect the fault and accomplish interruption of the circuit is also called a "ground fault sensor", or "ground fault sensor system."
One of the requirements of a ground fault sensor is that it functions rapidly so that a person or animal through which the fault circuit extends will not suffer a serious electrical shock. The hazard is a function of current magnitude and time or the integral of current.
This description of the background of the invention has emphasized ground faults through people and animals that occur in electrical appliances because providing an improved apparatus and method for doing that is one of the primary objects of the invention. However, there is a need in other circumstances for interruption of electrical power prior to the flow of excessive current on the occasion of an unwanted or unplanned ground fault and the invention is useful in those other circumstances as well.
While many schemes and circuits and devices have been proposed in the literature, it appears that attempts to produce ground fault sensors have been limited to those circuits and schemes in which ground fault is sensed as the difference in current flow in the two power lines, or as a difference in potential in those two power lines relative to some third reference. This invention is particularly useful with that kind of sensor. However, in a broader sense, the invention is useful with any kind of sensor that can detect a ground fault and provide a signal which can be converted to, or can be represented by, a flow of electrical current.
It is an object of the invention to provide a ground fault tester detector that can employ any kind of fault sensor whose output signal is, or can be converted to, an electrical current.
Many communities throughout the United States have adopted electrical codes that require the installation of ground fault sensors in the lines that supply power to swimming pools and electrical appliances of the kind that have proven to offer a high risk of electrical shock. Proposals to require greater use of ground fault sensors have been critisized because of the high cost of the sensors that are available. If cost and size were reduced sufficiently, ground fault sensors might be required at every industrial and commercial electrical outlet, and even in every residential electrical outlet. It is an object of this invention to provide a ground fault sensor which can be produced at low cost and in such small physical size that that objective can be realized.
A major obstacle to the development of low cost, miniaturized ground fault sensors has been inability of the sensor to distinguish between ground faults against which protection is required and electrical transients against which no protection is required. Much of the effort in ground fault sensor development has been devoted to an attempt to distinguish between a differential current in the power lines that is occasioned by a ground fault from a transient differential. It is not satisfactory to have the ground fault sensor shut down the supply of power to a food freezer or to a life saving apparatus in a hospital, and in many other instances when no fault is involved. Accordingly, ability to distinguish faults from mere transients is an essential requirement in a practical ground fault sensor.